This invention relates to printing machines and to vacuum drum assemblies for printing machines, such as inkjet or laser printers.
It is known for the printing drum in a printing machine to employ a vacuum to hold the paper or other material down on the drum. Such a drum might have an array of holes or passageways distributed along its length and around its periphery to permit air to flow from outside the drum to inside the drum in response to reduced air pressure inside the drum. In operation, a new sheet is fed to the rotating drum by a sheet feeder, and the vacuum captures it and rolls it on to the drum. As the drum and paper rotate, the paper passes one or more print heads which are used to print on the paper with as many revolutions as is necessary. As soon as the leading edge of the paper passes the print head, or last print head, on its last pass, an ejector is used to remove the paper from the drum. As soon as the trailing edge of the paper has passed the sheet feeder, the next sheet of paper is fed.
A problem which arises with such an arrangement is that, before the first sheet is fed, all of the holes or passageways in the drum are open, and therefore there is a large flow of air through the holes or passageways into the drum. Once a sheet is wrapped around the drum, some or all of the drum surface is closed, and a much lower flow of air is required. Particularly at the leading and trailing edges of the paper, its stiffness works against the vacuum. If a low density of suction holes is provided, these edges may then be released inadvertently. Accordingly, the total area of the holes or passageways needs to be as large as possible. However, a large area means that, in the case where no paper is loaded, a large volume flow is required to achieve a sufficient pressure differential. This requires a large fan, is noisy, and produces a loud slapping noise when paper is fed. There is also the related problem that the maximum flow obtainable may be determined primarily by the relatively smaller flow area presented by the end of the drum. Much of the power of the fan is dissipated in overcoming the pressure loss through this section, rather than producing a useful pressure differential at the drum surface.
In accordance with a first aspect of the present invention, there is provided a vacuum drum assembly for a printing machine, comprising:
a drum having an array of passageways distributed along its length and around its periphery to permit air flow from outside the drum to inside the drum in response to reduced air pressure inside the drum, and an array of valve members, each valve member being moveable between a closed position in which that valve member restricts at least one of the passageways and an open position in which the restriction of the at least one passageway is reduced;
the arrangement being such that, when a partial area of the drum is wrapped with material to be printed, at least some of the valves for the passageways adjacent an edge of that area are open, and the valves for the passageways which are not covered by the material and are not adjacent an edge of that area are closed.
A said valve member may be normally closed and may be opened by a pressure difference, for example between adjacent passageways.
In one embodiment, each passageway is provided with a respective such valve member. Each passageway could then be provided with a sensor for detecting, for example, the air pressure in that passageway upstream of the valve, or the air flow rate through the passageway, and the valve could be opened and closed in dependence upon the output of the sensor. Although possible, this would be a complicated arrangement.
In another embodiment, each of the valve members affects an adjacent pair of the passageways. In this case, the valve can be opened and closed automatically as a result of an imbalance or a balance of the pressures in the pair of passageways.
A particularly elegant and easily manufactured arrangement is possible when there are wall portions between adjacent pairs of the passageways, and each valve member comprises a butterfly valve pivotally mounted on a respective one of the wall portions and biased towards its closed position.
The term xe2x80x9cpivotally mountedxe2x80x9d is not intended to be limited to pin-jointed structure. It includes also arrangements in which the butterfly valve can tilt or rock about its (usually central) portion whereat it is attached to the wall portion.
In accordance with a further embodiment of the invention, each passageway is provided with a valve member which may be opened by mechanical actuation; for example, the valve member may include actuating means which moves the valve member to the open position on mechanical contact with the material to be printed. In a preferred form of this embodiment, the actuating means comprises a portion of the valve member which is housed within the passageway and is dimensioned to be proud of the drum when the valve member is in the closed position so that, in service, the material to be printed, as it is fed to the drum, urges the actuating means into the passageway thereby moving the valve member to the open position.
Preferably, the valve member is biased, suitably by resilient means, so that on removal of the material it moves back to the closed position. Alternatively, the valve member may be bistable; that is it may be biased towards closed when close to the closed position (thereby achieving good sealing) and also biased towards open when close to the open position: this is particularly useful for valve members near the edge of the material to be printed. A particular advantage of using a bistable valve in this context is that it ensures a fully open valve proximate the edge of the material to be printed. This is desirable since partial actuation of a valve (which might otherwise occur) may give rise to imperfect retention.
The resilient means used to bias may have a non-linear response.
Such an embodiment (unlike the above-mentioned embodiment wherein each valve member comprises a butterfly valve pivotally mounted on one of the wall portions and biased to its closed position) ensures that all such valve members remain in the open position until the removal of the material from mechanical contact with the actuating means; the material is thus held to the drum more definitely which facilitates print definition, particularly in multiple pass printing.
Each passageway may have a circular, annular, elliptic or polygonal, suitably a regular polygonal, cross-section and the passageways may be arranged as a tessellation. The cross-section of each passageway is preferably square, although other cross-sectional shapes may be employed, such as triangular and hexagonal.
The tessellation may be such as to provide rows of passageways generally parallel to the drum axis. It is preferred, however, that the rows are skew to the drum axis; this will ensure that the leading and trailing edges of the material to be printed fall at least on some valves, thereby facilitating its capture.
The curvature of the external surface of the drum about each passageway may be uniform; however, the external surface of the drum about each passage way may be flat or afford a spherical or cylindrical depression about the passageway, thereby increasing the area over which the vacuum from each passageway can act on the material to be printed and again facilitating its capture and retention.
There may be means for damping movement of the valve members. Thus the wall portions may be of energy-absorbing material and may be connected to the butterfly valves to effect said damping.
In a further embodiment, there is provided material stripping means positioned within the drum and actuatable to be urged into contact with the inside of the drum wall thereby moving all contacted valve members from the open position to the closed position. Preferably the stripping means is parallel with the drum axis and, suitably, coextensive with the length of the drum. In a preferred form of the embodiment the stripping means comprises a non-driven, but rotatable, cylinder and is suitably mounted at a station where the leading edge of the printed material is, after its final pass, required to be stripped.
In accordance with a second aspect of the present invention, there is provided a printing machine, including a vacuum drum assembly according to the first aspect of the invention.
A third aspect of the present invention is concerned with duplex printing machines, that is machines which can print on both sides of a sheet of material. It is known to provide inkjet and laser printers with a duplexing facility, for example by printing on one side of the material and then reversing the direction of feeding of the material and diverting its path so that it returns to the printing position effectively turned over.
In the accordance with a third aspect of the present invention, there is provided a duplex printing machine comprising: first and second vacuum drum assemblies each in accordance with the first aspect of the invention and with their drums parallel; means for reducing the air pressure inside the drums; means for counter-rotating the drums; means for feeding a sheet of material to be printed on to the first drum so that the material can be held on the first drum by vacuum and rotated therewith; first printing means for printing on the material on the first drum; means for releasing the material from the first drum in a direction towards the second drum so that the material can be held on the second drum by vacuum and rotated therewith; second printing means for printing on the material on the second drum; and means for releasing the material from the second drum. Although such a machine uses two drums and two printing means, it provides a very neat and compact arrangement.
Since the vacuum drum assemblies as described above will have less of a tendency violently to grab the leading edge of the material being fed onto them, the machines of the second and third aspects of the invention preferably further include, for the or each drum, means for holding or directing the material against or towards the or the respective drum at the position in which the material is fed on to the, or the respective, drum. These means may for example be a pinch roller or guide.
This roller or guide optionally may press intermittently on to the drum, e.g. only when a sheet of material is being fed on to the drum, and then withdraw. This will reduce any tendency for the roller or guide to offset (transfer) still-wet ink from the sheet onto another portion thereof, or on to a subsequent sheet.
The duplex configuration of drums may be provided independently of the first aspect of the invention. Therefore, in accordance with a fourth aspect of the present invention, there is provided a duplex printing machine comprising: first and second parallel drums; means for counter-rotating the drums; means for feeding a sheet of material to be printed on to the first drum; means for holding the fed material on the first drum; first printing means for printing on the material on the first drum; means for releasing the material from the first drum in a direction towards the second drum; means for holding the fed material on the second drum so as to be rotated therewith; second printing means for printing on the material on the second drum; and means for releasing the material from the second drum.
In one embodiment, the direction in which the material is released from the first drum towards the second drum is generally parallel to and opposite to the direction in which the material is fed onto the first drum; and the direction in which the material is released from the second drum is generally parallel to and opposite to the direction in which the material is fed onto the second drum. Said directions may be generally horizontal, if the sheets are stacked horizontally or vertical if the sheets are stacked vertically.